Chemically, it is a thioether with the formula C4H8Cl2S. Its structure can be described as 1,1,thiobis[2,chloroethane] or (ClCH2CH2)2S or 2,2'-dichlorodiethyl sulfide or bis-(2-chloroethyl)sulfide). The compound reacts with itself and water to form hydrocholirc acid. The electron rich sulfur attacks one of the carbons attached to a chlorine. Water then attacks the sulfonium ion ring that has formed opening the ring. The end result is a 'substitution' of the chlorine atom with an alcohol and a molecule of HCl. The sulfonium ion ring is particularly detrimental to cellular health as it has a strong tendency to bond to the guanine nitrogen in DNA strands. This leads to either immediate cellular death or, as recent research has found, cancer. Mustard is not very soluble in water but is very soluble in fat, contributing to its rapid absorption into the skin.
Mustard gas is a strong vesicant (a compound that causes blisters). Those exposed suffer immediate itching which develops (in 4 to 24 hours) into deep blisters on the skin, the eyes become sore and the eyelids swollen, possibly leading to conjunctivitis and blindness. At very high concentrations, if inhaled, it causes bleeding and blistering within the respiratory system, damaging the mucous membrane and leading to pulmonary edema.
Skin damage can be reduced if povidone iodine in a base of glycofurol is rapidly applied. The vesicant property can be neutralised by oxidation or chlorination, common bleach (NaOCl-) can be used or decontamination solution "DS2" (2% NaOH, 70% diethylenetriamine, 28% ethylene glycol monomethyl ether). Mustard gas is probably also carcinogenic (cancer causing).
It was first used by the German Army against Canadian soldiers in 1917 and later also against the French. It was dispersed as an aerosol in a mixture with other chemicals, giving it a yellow-brown colour and a distinctive odour. Mustard gas was lethal in only about 1% of cases. Its effectiveness was as an incapacitating agent: a wounded soldier slows an advancing army much more than a dead one.
Most of the mustard gas found in Germany after World War II was dumped into the Baltic Sea. It is possible to mistake a piece of polymerised mustard gas for amber, which can lead to severe health problems. Shells containing mustard gas and other toxic ammunition from World War I (as well as conventional explosives) can still occasionally be found in France; they used to be disposed of by explosion at sea, but current environmental regulations prohibit this; the French government is building an automated factory to dispose of the backlog of shells.